What he said , riding the 318 from Shanghai to Lhasa is a right of passage for Young Chinese lads and lasses. The western part of it is good to ride in parts but for sure there are lots of better roads with way less traffic.

Road to Hell would be the G219(actually G219 is famed as Road paved on the heaven but most people said it was a hell of a trip) if you make it to Lhatse take the G219 to Yecheng might take you a while to do it on a push bike though. It's all tar now I'm told but I wonder. Heard of people doing it on push bikes many years ago while it was dirt road.Hats off to them. You can take a detour up the northern end and try to get to K2 base camp not many people accomplish it though for good reason.

The best start point IMO for the G318 Ya'an from here you can head west with roads passes close to 5000 meter high but due to special permits to entry Tibet I only got as far as the border town of Zhubalongcun, cross over the river Jinsha and you are in Tibet.
still over 600 km of some of the best demanding roads you could wish to ride , along the way you pass close to Zong Shan mountain at 6885 meters high.
But I am told the best bits of the S318 is after the border crossing between Bangdazhen and Nyingchi and just around this area you pass Namcha Barwa mountain at 7782 meters high.